1. Field of the Invention
The present invention relates generally to an exhaust emission control system of an internal combustion engine and, more particularly, to an exhaust emission control system of an internal combustion engine which is capable of switching over a flow direction of an exhaust gas flowing through an exhaust gas purifying element according to the necessity.
2. Related Background Art
In general, an exhaust emission control system for purifying an exhaust gas discharged from an internal combustion engine is provided at an exhaust passageway of the internal combustion engine. When the exhaust gas from the internal combustion engine flows through this exhaust emission control system, a deposit is gradually adhered from an upstream side in the exhaust emission control system. A classification of what this deposit is all about might differ depending upon a composition of the exhaust gas, or a construction of the exhaust emission control system or an exhaust gas purifying mechanism, and what can be exemplified as the deposit may be, e.g., an oxide, a sulfide, nitrate and sulfate. This deposit might cause a decline of a purging performance of the exhaust emission control system and also an increase in an exhaust resistance, and therefore needs to be removed at a predetermined timing.
For example, the exhaust emission control system for purging the exhaust gas of NO.sub.x which is discharged from the internal combustion engine for performing a combustion at a lean air/fuel ratio, may involve the use of an NO.sub.x storage-reduction catalyst. This NO.sub.x storage-reduction catalyst absorbs NO.sub.x when the air/fuel ratio of the inflow exhaust gas is lean, and desorbs NO.sub.x absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases, thus effecting reduction to N.sub.2. The NO.sub.x storage-reduction catalyst is disposed in an exhaust passageway, and absorbs a nitrogen oxide (NO.sub.x) contained in the exhaust gas exhibiting the lean air/fuel ratio. After absorbing NO.sub.x, the air/fuel ratio of the exhaust gas flowing into the NO.sub.x storage-reduction catalyst is made rich by increasing a quantity of the fuel supplied to the internal combustion engine, thereby desorbing NO.sub.x absorbed thereto from the NO.sub.x storage-reduction catalyst. Desorbed NO.sub.x is reduced to N.sub.2 with a reducing component such as unburned HC, CO etc contained in the exhaust gas.
By the way, generally speaking, the fuel of the internal combustion engine contains a sulfur content, and, when the fuel is burned in the internal combustion engine, the sulfur content is burned, resulting in a production of sulfur oxide (SO.sub.x). The NO.sub.x storage-reduction catalyst absorbs SOX contained in the exhaust gas with the same mechanism as absorbing NO.sub.x. Therefore, if the NO.sub.x storage-reduction catalyst is disposed in the exhaust passageway of the internal combustion engine, the NO.sub.x storage-reduction catalyst absorbs SO.sub.x as well as NO.sub.x.
SO.sub.x absorbed to the NO.sub.x storage-reduction catalyst, however, generates stable sulfate with a passage of time. SO.sub.x is therefore dissolved and desorbed with a difficulty and has a tendency of being easily accumulated within the NO.sub.x storage-reduction catalyst under a condition of executing desorption, reduction and purging of NO.sub.x out of the NO.sub.x storage-reduction catalyst (which is hereinafter termed a NO.sub.x desorbing/reducing process). If there augments a SO.sub.x accumulation quantity within the NO.sub.x storage-reduction catalyst, a NO.sub.x absorption quantity of the NO.sub.x storage-reduction catalyst decreases, and it is therefore unfeasible to sufficiently purge the exhaust gas of NO.sub.x, with the result that so-called SO.sub.x poisoning occurs, wherein a NO.sub.x purging rate declines. Such being the case, it is required that SO.sub.x absorbed to the catalyst be desorbed therefrom at a proper timing in order to keep high the NO.sub.x purging rate of the NO.sub.x storage-reduction catalyst for a long period of time.
It has already proved that the air/fuel ratio of the inflow exhaust gas needs to be rich and the NO.sub.x storage-reduction catalyst is required to be set at a higher temperature than in the NO.sub.x desorbing/reducing process for desorbing SO.sub.x absorbed to the NO.sub.x storage-reduction catalyst.
Incidentally, a distribution of a SO.sub.x absorption quantity in the NO.sub.x storage-reduction catalyst exhibits a higher concentration in the closer proximity to the inlet of the exhaust gas in the NO.sub.x storage-reduction catalyst. Hence, when desorbing SO.sub.x absorbed to the NO.sub.x storage-reduction catalyst, in the case of flowing the exhaust gas having the rich air/fuel ratio in the same direction as a flow direction of the exhaust gas when absorbing NO.sub.x, though SO.sub.x absorbed is desorbed on the inlet side in the NO.sub.x storage-reduction catalyst, SO.sub.x desorbed therefrom merely migrates to the outlet side of the exhaust gas through the NO.sub.x storage-reduction catalyst and is reabsorbed to the NO.sub.x storage-reduction catalyst. The problem is therefore such that SO.sub.x can not be discharged from the NO.sub.x storage-reduction catalyst.
Under such circumstances, a technology disclosed in Japanese Patent Application Laid-Open Publication No.7-259542, is that when desorbing SO.sub.x absorbed to the NO.sub.x storage-reduction catalyst, the exhaust gas having the rich air/fuel ratio flows through the NO.sub.x storage-reduction catalyst in a direction opposite to the direction when absorbing NO.sub.x. In the case of incorporating a backward flow function of desorbing SO.sub.x by reversing the flow of the exhaust gas as described above, SO.sub.x desorbed from the NO.sub.x storage-reduction catalyst has a shorter migration distance within the NO.sub.x storage-reduction catalyst, and is immediately discharged out of the NO.sub.x storage-reduction catalyst. It is therefore feasible to prevent desorbed SO.sub.x from being reabsorbed to the NO.sub.x storage-reduction catalyst.
The following is an explanation of a construction of the exhaust mission control system of the internal combustion engine which incorporates the backward flow function disclosed in the above Publication. An upstream-side exhaust passageway connected to an inlet of the NO.sub.x storage-reduction catalyst is connected to a downstream-side exhaust gas passageway connected to an outlet of the NO.sub.x storage-reduction catalyst via a bypass passageway for bypassing the NO.sub.x storage-reduction catalyst. A first emission flow switching valve is provided at a confluent portion between the upstream-side exhaust passageway and the bypass passageway. A second emission flow switching valve is provided at a confluent portion between the downstream-side exhaust passageway and the bypass passageway. The first emission flow switching valve is capable of making a changeover to let the exhaust gas flowing from upstream flow through the NO.sub.x storage-reduction catalyst or let the exhaust gas flow into the bypass passageway. The second emission flow switching valve is capable of making a changeover to let the exhaust gas flowing through the NO.sub.x storage-reduction catalyst flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve or let the exhaust gas flowing though the bypass passageway flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve. Further, an exhaust passageway for suction is bypassed from the upstream-side exhaust passageway between the NO.sub.x storage-reduction catalyst and the first emission flow switching valve, and is connected to an intake port of an exhaust pump, and a discharge port of the exhaust pump is connected to the above bypass passageway. Moreover, a reducing agent supply device for supplying a reducing agent is provided at the downstream-side exhaust passageway between the NO.sub.x storage-reduction catalyst and the second emission flow switching valve.
Then, when in the NO.sub.x absorbing process, the first and second emission flow switching valves are switched over to close the bypass passageway so that an entire quantity of exhaust gas from the internal combustion engine flows toward the outlet from the inlet of the NO.sub.x storage-reduction catalyst. On the other hand, when desorbing SO.sub.x from the NO.sub.x storage-reduction catalyst, the first and second emission flow switching valves are switched over to close the bypass passageway so that substantially the entire quantity of exhaust gas from the internal combustion engine flows to the bypass passageway. Simultaneously, the exhaust gas in the upstream-side exhaust passageway between the NO.sub.x storage-reduction catalyst and the first emission flow switching valve is sucked and discharged to the bypass passageway by operating the exhaust pump, thereby causing a flow of the exhaust gas flowing backward from the outlet toward the inlet through the NO.sub.x storage-reduction catalyst. Besides, the reducing agent is supplied to the downstream-side exhaust passageway by operating a reducing agent supply device. The exhaust gas exhibiting the rich air/fuel ratio is thereby flowed backward through the NO.sub.x storage-reduction catalyst, thus desorbing SO.sub.x from the NO.sub.x storage-reduction catalyst.
The conventional backward-flow-function-incorporated exhaust emission control system of the internal combustion engine requires the exhaust pump and the plurality of emission flow switching valves and therefore involves the use of an increased number of parts, resulting in a rise in costs. Further, the increased number of parts leads to a good deal of labors for maintenance and inspection, correspondingly.
Moreover, the SO.sub.x desorbing process involving the backward flow in the NO.sub.x storage-reduction catalyst may be defined as a processing method giving attention to avoidance of the SO.sub.x reabsorption by decreasing the SO.sub.x migration distance when desorbing SO.sub.x. While on the other hand, if this processing method is adopted, there increases a distance till the exhaust gas arrives at the NO.sub.x storage-reduction catalyst, and hence there must be a large drop in temperature of the exhaust gas during a period for which the exhaust gas flows through this long route. The above processing method is not necessarily, as the case may be, considered the best method of desorbing SO.sub.x in terms of a temperature condition when desorbing SO.sub.x.